The present invention relates to a microfilm copying apparatus for magnifying and copying microfilm images.
There are known microfilm copying apparatus for magnifying and copying microfilm images. Typical prior optical systems for exposing photosensitive bodies to light through slits in the known microfilm copying apparatus include arrangements shown in FIGS. 7 and 8.
In the optical arrangement shown in FIG. 7, the image on a microfilm 181 is projected by a projection lens 182 onto a photosensitive body 184 at a magnification m (=b/a). The projection lens 182 has an optical axis aligned with the optical axis of an illuminating optical system (not shown). While the projection lens 182 and the illuminating optical system are kept at rest in the apparatus, the microfilm 181 and the photosensitive body 184 are displaced in the directions of the illustrated arrows at speeds VF, VP, respectively. The speeds VF, VP meet the relationship VF =mVP. The distance which the microfilm 181 is displaced is equal to the sum SF of the size Io of the microfilm image and the width WF of a slit through which light from the illuminating optical system is passed toward the microfilm 181. The slit width WF is equal to WS/m where WS is the width of a slit through which light from the projection lens 182 is passed toward the photosensitive body 184, and which is defined by light shielding plates 185, 186.
The magnification m is generally in the range of from 10 to 20, and the slit width WS generally ranges from 5 to 10 mm. In the system of FIG. 7, therefore, a half angle .theta..sub.2 of view for slit exposure is at most 0.5.degree., and any exposure irregularity or imperfection on the photosensitive body 184 in the direction transversely across the exposure slit is at most 1% or substantially nil.
Microfilms are available in various formats such as aperture cards and roll films, and in different sizes ranging from small sizes up to microfiche in A6 size. To make the microfilm copying apparatus usable for different microfilm formats and sizes, it is required to have different microfilm holders and convey them for exposure. As disclosed in Japanese Laid-Open Patent Publication No. 59(1984)-30527, the microfilm holders are conveyed by a step motor, which is highly advantageous in that it can easily meet various magnifications. However, the step motor is not capable of cope with a variety of load variations, and the speed of movement of the microholders which are different in formats and weights cannot be controlled by the step motor.
The slit exposure system shown in FIG. 8 is designed to eliminate the aforesaid problem. The image on a microfilm 181 is projected through a slit onto a photosensitive body 189 through a projection lens 183 and mirrors 187, 188.
The microfilm 181 and the projection lens 183 are held at rest in the microfilm copying apparatus. The photosensitive body 189 is exposed to light by moving the mirrors 187, 188 in unison in the direction of the arrow at a speed VM in synchronism with the movement of the surface of the photosensitive body 189 at a speed VP. The slit through which the light from the mirrors 187, 188 passes toward the photosensitive body 189 is defined by a pair of light shielding plates 190, 191. If an angle .phi. is formed between optical axes X, Y, then the speed VM of movement of the mirrors 187, 188 is expressed by VP/2 cos(.phi./2). If the optical axes X, Y are parallel to each other (.phi.=0) as shown in FIG. 8, then the speed VM is given by VP/2. The distance SM that the mirrors 187, 188 travel is generally expressed by: ##EQU1## The distance SM in FIG. 8 (.phi.=0) is: EQU SM=mIo/2
where Io is the size of the microfilm image and m is the magnification.
Since only the mirrors 187, 188 are movable irrespectively of the format and weight of the microfilm holder used, this exposure system is free of the problem described with reference to FIG. 7. The half angle .theta..sub.3 of view required for slit exposure through the projection lens 183 is however in the range of from 15.degree. to 20.degree.. Because of the cosine law, the illuminance in the direction of travel of the photosensitive body 189 varies up to 20% as the mirrors 187, 188 are displaced. To produce good copied images, the microfilm copying apparatus must be equipped with a dedicated corrective means for correcting such illuminance variations, and hence is complex in structure.
Where a microfilm copying apparatus is constructed as a microfilm reader-printer, a screen housing unit holding a screen is generally positioned in an upper portion of the apparatus, and a copying mechanism including a photosensitive body and an optical system for leading a focused beam of light to the photosensitive body are generally positioned in a lower portion of the apparatus and disposed in an apparatus base on which the screen housing unit is supported.
In known microfilm reader-printers, the screen and the screen housing unit have to be disassembled and detached when the optical system or other mechanisms are to be repaired, maintained, or cleaned. It is therefore tedious and time-consuming to operate upon the microfilm reader-printers for repair, maintenance, or cleaning.
The prior microfilm reader-printers employ one common focusing lens for both displaying a microfilm image on the screen and copying the microfilm image. In general, therefore, a first exposure mirror is angularly movable between a position in which the mirror is retracted from the focused beam of light when the image is to be displayed on the screen and another position in which the mirror is in the focused beam of light. When the first exposure mirror is to be moved into the focused beam of light, external light passing through the screen falls on the first exposure mirror and is reflected thereby as stray light which adversely affects the copying process. Inasmuch as positioning means and light shielding means for the first exposure mirror are separate from each other, the overall structure is complex and highly costly.